1. Field of the Invention
The present invention relates to X-ray lithographic systems, and more particularly to an X-ray lithographic system which can transfer a mask pattern of large area uniformly at high precision.
2. Description of the Prior Art
X-ray lithography is a method wherein, as shown in FIG. 1, a mask 2 is irradiated with X-rays 1, and a mask pattern made up of an X-ray transmissive material 3 and an X-ray absorptive material 4 is transferred on a radiation sensitive resist film 5 located behind the mask. Usually the X-rays obtained from the point X-ray source of an electron beam excitation type rotating anode X-ray tube are utilized, but they have the problems that the intensity is low and that the resolution degrades due to penumbra blurring ascribable to a divergent X-ray flux. In contrast, synchrotron radiation is about 10.sup.3 times higher in intensity than the point X-ray source and exhibits a good collimation. A synchrotron is therefore one of the most hopeful X-ray sources in transferring a pattern in the order of submicrons.
Although the cross section of a synchrotron radiation flux somewhat differs depending upon the scale of the synchrotron, it is in the shape of an ellipse having a minor axis of approximately 20-30 mm and a major axis of approximately 40-60 mm at a position which is about 10 m distant from a synchrotron orbit. Since, however, there is an intensity distribution in a direction perpendicular to the plane of the synchrotron orbit, an area over which a resist film can be exposed to the X-rays effectively uniformly is merely a band-like area having a width of several mm, and a problem is involved in the exposure of a silicon wafer of large diameter.
In order to solve the problem, it is considered that mounting stages for a mask and the wafer are moved relative to the synchrotron radiation, to equivalently scan the surfaces of the mask and wafer with the radiation. In this case, however, such new problems arise that mechanisms for positioning the mask and wafer and mechanisms for moving the mask and wafer mounting stages are individually required, resulting in a complicated system structure, and that the mask and wafer are vibrated by the operations of the moving mechanisms during the exposure, making a precise pattern transfer impossible.
To the end of avoiding the aforementioned problem, there has been proposed a method wherein, as shown in FIG. 2, synchrotron radiation 32 is reflected by a convex mirror 31 so as to enlarge the cross-sectional area of a radiating flux 33 (IBM Research Report, RC8220, 1980). However, the synchrotron radiation is a continuous spectrum as shown in FIG. 3, and according to this method, a wavelength component which has a critical glancing angle smaller than the angle of incidence .theta..sub.A of a flux A to the mirror 31 in FIG. 2 is hardly projected on a specimen surface S.sub.A because the reflection factor becomes a very small value. On the other hand, the angle of incidence .theta..sub.B of a flux B to the mirror is .theta..sub.B &lt;.theta..sub.A, and a radiation flux of shorter wavelength is more projected on a specimen surface S.sub.B than on the specimen surface S.sub.A. Accordingly, a wavelength distribution arises in the radiation projected on a specimen, and the specimen cannot be uniformly exposed to the radiation.
Further, even in a case where, in the above method employing the convex mirror, the wavelength distribution of the radiation projected on the specimen is reduced by the use of monochromatized radiation, the reflected lights of equal flux widths .DELTA. in the fluxes A and B are magnified with unequal factors, and the reflection factor has the incidence angle-dependency even within the critical glancing angle, so that the intensity distribution is still involved in the radiation projected on the specimen, and uniform exposure over the whole specimen surface cannot be performed.